One method to increase the amount of bandwidth in a fiber optic communication system is to transmit multiple streams of information-bearing data on different wavelengths in the same physical optical transmission system. This technique is referred to as wavelength division multiplexing (“WDM”). Implementations of this technique involving a higher number of densely packed channels are referred to as dense wavelength division multiplexing (“DWDM”). Prior-art examples of DWDM networks consist of multiple wavelength channels occupying the 1530–1560 nm (C-band) and 1565–1605 nm (L-band) ranges. They support either 40 channels spaced 0.8 nm (100 GHZ) apart or 80 channels spaced 0.4 nm (50GHz) apart, and are transmitted on the same physical fiber optic transmission cable.
One of the basic components required to steer each data-bearing wavelength channel through the DWDM network is an optical filter. Known examples of optical filters include Fabry-Perot etalons, dielectric thin film filters, arrayed waveguide gratings (“AWG”s), and fiber Bragg gratings.
Filters will have many possible uses in future WDM and DWDM networks such as the following:                A filter placed in front of an incoherent receiver can be used to select a particular signal from many arriving signals.        A filter placed in front of a broadband transmitter such as an LED can be used to select a particular wavelength in which to modulate data.        WDM networks are proposed which use filters to control which path through a network a signal will take.        
There are disadvantages with the known DWDM filters, primarily in terms of cost, size, and functionality. Most commercially available DWDM filters are of the dielectric thin film type. In order to get smaller and smaller bandwidths that next generating DWDM networks are demanding using these filters, more and more thin film layers are required. Because each thin film deposition step is an independent process, the cost increases as more layers are added. Further, the product yield decreases due to the fact that every time a new layer is deposited there is some probability of an error occurring.
Based on the foregoing, there is a need for an improved WDM or DWDM filter.